As well as not having an upstream 02 sensor.

The computer cannot mix properly without all of its sensors. I went through this today with a bad 02 sensor. Upstream drivers side went, computer thought left bank was lean, so it dumped all the fuel it could into it, fuel fouled all the plugs.

 

truck backfires through air cleaner while driving idles fine then straightens out. replaced plugs injectors wires still does it. code comes up #3 cyl. miss firing. light don't always come on. changed crank shaft sensor, still does it seems worse when fuel gets below 1/4 of tank.

 

If you're getting a mix of backfire (exhaust) and coughing (intake), I would replace the crank position sensor... especially if your anywhere near 100K.